The present invention concerns an antiskid control circuit for individually regulating the braking pressure at the steering wheels of a motor vehicle.
On roadways with different coefficients of friction (.mu.-values) for the left and right wheels of an axle (split coefficients of friction), different coefficient of friction/slip curves (.mu./.lambda.-curves) are obtained for the two sides of the vehicle. Under these conditions differential brake pressure arises during braking, which results in high yawing moments and steering forces, and this reduces the steering stability.
So-called "select low" control is well known. In "select-low" control, the wheel running at the higher .mu.-value is co-controlled in accordance with the regulated braking by the control circuit of the wheel running at the lower .mu.-valve. This has the advantage of reducing the variable steering forces that arise with split coefficients of friction, but the braking distance is unacceptably long.
In regard to antiskid brake systems in which the control circuits of the wheels of one axle are connected with each other in accordance with a "select-low" control system and in which provision is made for co-control of the still unregulated wheel brake pressure of an uncontrolled wheel by the control circuit of a controlled wheel at a given moment of time, it is already known to delay for a predetermined time the brake pressure regulation of the uncontrolled wheel. However, the brake pressure for the wheel running at the higher .mu.-value (high adhesion wheel) is reduced within the period that a deceleration control signal (-b signal) in the control circuit of the wheel running at the lower .mu.-value (low adhesion wheel) (FIG. 3c) is still present. In this way the braking force and thus the braking moment are still reduced too soon, i.e., while the high adhesion wheel is in the stable range. At the low adhesion wheel, however, the braking moment is maintained longer, despite the brake pressure reduction, and is first removed when the low wheel has returned from the unstable range to the stable range through the critical .mu.-value of the associated .mu./.lambda. curve (FIG. 3d). In this way, despite the advantages that the known system has over comparable systems, there is the danger that yawing moments and appreciable steering forces will arise because the left and right wheels on the same axle can have different brake forces at the same time.